1. Field of the Invention
This invention relates to plasma arc welding mills in which titanium tube preforms are drawn and closed under a relatively stationary plasma arc welding torch. The preforms are drawn through a series of capstan rollers, past a knife-edge guide bar and under the plasma arc torch where a key-hold butt weld is made along the longitudinal seam. The weld area of the tube and some distance down stream of the weld is shielded by an inert gas, usually argon, confined within an upper box-like trailer carried by the frame of the mill and riding on the outer surface of the tube above the weld and a similar lower trailer supported on a mandrel within the tube which is carried by the guide bar and rides on the inside wall of the tube beneath the weld.
2. Description of the Prior Art
The U.S. Pat. to Paul M. Erlandson No. 3,652,818, issued Mar. 28, 1972 shows an inert atmosphere seam welder for resistance welding of the overlapping edges of can bodies. This patent also shows upper and lower gas confining trailers having flexible edges engaging the outer and inner tubular wall of the can in the vicinity of the welding torch.